1. Technical Field
The present invention relates to a power generation system for causing a reaction of anode gas with cathode gas in a fuel cell for power generation. More particularly, the present invention relates to a power generation system using molten carbonate fuel cells, in which sensible heat of cathode exhaust gas is utilized as part of heat source for a reformer when fuel gas is reformed into anode gas.
2. Background Art
A molten carbonate fuel cell includes a plurality of cell elements stacked with separator plates being interposed between the cell elements. Each cell element includes an electrolyte plate (a porous plate soaked with molten carbonate), a cathode (oxygen electrode) and an anode (fuel electrode). The electrolyte plate is sandwiched by these electrodes. In the fuel cell, cathode passages and anode passages are formed to respectively feed anode gas to an anode chamber and cathode gas to a cathode to perform power generation.
A conventional molten carbonate fuel cell is illustrated in FIG. 15 of the accompanying drawings. In FIG. 15, "I" indicates a fuel cell, and a cathode chamber 2 feeds cathode gas CG to a cathode and an anode chamber 3 feeds anode gas AG to an anode. Numeral 10 designates a reformer which reforms fuel gas such as natural gas. The reformer 10 includes a combustion chamber 10a and a reforming reaction tube 10b extending through the combustion chamber 10a.
First, feeding of cathode gas to the cathode chamber 2 of the fuel cell I will be described. Air A is pressurized by a compressor 4, driven by gas G then cooled by a cooling device 5 and compressed again by another compressor 6. After that, the air is preheated by an air preheater 7 and fed into a cathode chamber 2 through a line 8 with exhaust gas from the combustion chamber 10a of the reformer 10 as well as cathode recirculation gas from a recirculation line 31. Also, part of the air preheated by the air preheater 7 is fed to the combustion chamber 10a of the reformer 10 through a line 9. Cathode exhaust gas discharged from the cathode chamber 2 is not only recirculated to the cathode chamber 2 via the recirculation line 31 but also introduced to a turbine 12 through a line 11 and then expelled to atmosphere via the air preheater 7 and a water heater 21.
Next, the anode gas fed to the anode chamber 3 will be described. Natural gas NG passes through a preheater 14 and a desulfurizer 25 and then enters the reforming reaction tube 10b of the reformer 10 with steam supplied from a steam line 32. Natural gas is reformed in the reforming tube 10b to become the anode gas and then introduced to the anode chamber 3. Anode exhaust gas from the anode chamber 3 is cooled through a heat exchanger 13 and introduced to a cooling device 16 via the preheater 14 and the vaporizer 15. In the cooling device 16, the anode exhaust gas is condensed and the water thereof is separated from gas by a gas-liquid separator 17. Separated gas is sent to the heat exchanger 13 via the line 19 by means of the blower 18 driven by motor m and sent to the combustion chamber 10a of the reformer 10 in which unreacted H.sub.2 and CO are combusted with air fed from the line 9 to maintain a reforming temperature of the reforming tube 10b. On the other hand, water separated by the gas-liquid separator 17 flows through a line 32 to be compressed by a pump 20, to be heated by the water heater 21 and to be mixed with natural gas coming from the desulfurizer 25 through the line 22, the vaporizer 15 and the steam line 32. The the water is finally recirculated to the anode chamber 3.
Combustion exhaust gas of the combustion chamber 10a is fed through the line 24 to the cathode chamber 2 as the cathode gas.
In the power generation system of FIG. 15, the temperature maintenance of the reforming reaction tube 10b of the reformer 10 is influenced by an amount of heat of air fed into the combustion chamber 10a, an amount of heat of the anode exhaust gas which has been separated from the water and combustion heat of these gases. However, the anode exhaust gas is cooled at the water-gas separation process so that it is necessary to lower a fuel utilization factor in order to raise an amount of combustible gas among the anode exhaust gas and to increase the combustion heat. An example is illustrated in FIG. 17, in which Ti is assigned to an entrance temperature of the reformed gas of the reaction tube 10b, To is assigned to an exit temperature of the same, T.sub.1 to an entrance temperature of the combustion chamber 10b, T.sub.2 to the anode exhaust gas temperature, T.sub.3 to a combustion temperature after-the-combustion-temperature), T.sub.4 to a temperature at the exit (this temperature is lower than T.sub.3 due to the heating of the reforming section). In this case, the exit temperature T.sub.4 of the exhaust gas from the combustion chamber 10a is higher than the air temperature T.sub.1 at the entrance and the anode exhaust gas temperature T.sub.2. Therefore, an amount of heat required for the combustion includes heat for the reformation and heat for raising the air and the anode exhaust gas temperature to the exit temperature T.sub.4. In other words, some heat is wasted to raise the air temperature and the anode exhaust gas temperature. The same symbols A, G, M, AG and CG are utilized in the other figures of drawings herein with the same meanings as given hereinabove.
Referring to FIG. 16, there has been proposed a power generation system which maintains the reforming temperature of the reformer 28 by the sensible heat of the anode exhaust gas.
In FIG. 16, the natural gas NG is pressurized by the blower 26 and introduced to the preheater 26, the desulfurizer 25 and the reformer 28 (The reformer 28 has the reforming chamber only). After that, the natural gas is sent to the preheater 27 and the anode chamber 3 by the blower 29. Part of the anode exhaust gas from the anode chamber 3 is mixed with the natural gas and then introduced to the reformer 28 as the heat source for the reformer 28 whereas the remainder is introduced to the catalyst combustor 30.
Next, the cathode gas to be supplied to the cathode chamber 2 will be explained. The air is compressed by the compressor 4, cooled by the cooling device 5 and preheated by the air preheater 7. Then, the air is led to the catalyst combustor 30 through the line 8 and used to combust the combusible components among the anode exhaust gas fed into the catalyst combustor 30. Thereafter, the air is sent to the cathode chamber 2. The cathode exhaust gas from the cathode chamber 2 is partially recirculated to the cathode chamber 2 via the recirculation line 31 and partially introduced to the turbine 12 via the line 11 to be expelled to the atmosphere via the air preheater 7.
In the power generation system of FIG. 16, since the reforming temperature of the reformer 28 is maintained by the sensible heat of the anode exhaust gas, it is necessary that a large amount of anode exhaust gas is fed to the reformer 28. However, as a amount of anode exhaust gas to be supplied increases, the concentration of the fuel gas (H.sub.2 and CO) among the anode gas becomes leaner which results in deterioration of fuel performances and drop of power generation efficiency. In addition, since the reforming temperature of the reformer 28 becomes lower than the anode exhaust gas temperature at the anode chamber 3 exit, which is the fuel cell system operation temperature, the reforming ratio cannot be set high.